In process automation technology, field devices are often used, which serve to register and/or influence process variables. Examples of such field devices are fill level measuring devices, mass flow meters, pressure and temperature measuring devices, etc., which, as sensors, register the corresponding process variables, fill level, flow, e.g. flow rate, pressure and temperature, respectively.
Serving for influencing process variables are also field devices in the form of actuators, e.g. valves or pumps, via which flow of a liquid in a pipe section, and/or fill level in a container, can be changed.
In principle, all devices are designated as field devices, which are used near a process and which supply or process process-relevant information.
A large number of such field devices are manufactured and sold by the firm, Endress+Hauser.
As a rule, field devices in modern industrial plants are connected by bus systems (Profibus, Foundation Fieldbus, etc.) with superordinated units (control systems or control units). These superordinated units serve, among other things, for process control, process visualization, process monitoring, as well as for start-up of the field device.
Normally, the superordinated units are control units (e.g. programmable logic controllers, or PLCs). In the bus system (Profibus PA or DP), these control units function as Master Class 1. They are responsible for the cyclic data traffic on the bus system.
For the parametering and diagnosis of field devices, today, as a rule, additionally, service units with corresponding service programs (e.g. PDM of the firm, Siemens, FieldCare of the firm, Endress+Hauser) are deployed, which communicate with the field devices acyclically and function as Master Class 2.
Recently, the Profibus specifications were adapted so that also Master Class 1 units can make use of acyclic services. That means that parametering of field devices is now also possible with control units meeting the new specifications.
At present, in existing applications, almost exclusively, control units of the older generation are to be found, which have no acyclic services. Also in future applications, due to reliability reasons, control units of the older generation are apt still to be used frequently.
In order to be able to use the newly specified, acyclic services, the older control units would have to be replaced with new control units. For this, a shutdown of the plant would be necessary, accompanied by an interruption of the control process. Integration of the new control units could only be accomplished by highly-qualified technical personnel. All these measures would be extremely complex for the user and be tied to substantial costs. The probability that older controls will continue to be used into the future is, therefore, quite high.